Image forming apparatuses using electrophotographic image forming process include an image forming apparatus developing with a two-component developer formed of a toner and a carrier which is a magnetic particulate material and an image forming apparatus developing with a one-component developer formed of only a toner without including a carrier. Japanese published unexamined application No. 2005-338814 discloses an image forming apparatus forming an image with a two-component developer, in which a toner and a carrier are stirred in an image developer such that the toner is charged and the charge toner is transferred to an electrostatic latent image formed on a latent image bearer such as a photoreceptor to form a toner image (visual image). When the toner image is formed, only the toner is fed from the image developer to the latent image bearer. Accordingly, the toner in the image developer decreases with formation of the toner images and a ratio of the toner to the carrier in the two-component developer (a toner density) changes. Therefore, in the image forming apparatus using the two-component developer, a toner is fed by a toner feeder into the image developer.
When a toner is fed into the image developer too much and an amount of the toner in the two-component developer excessively increases, an individual toner has less opportunity to contact a carrier and is not sufficiently charged. This causes foggy images, i.e., a toner scatters on parts besides an image on a recording material. To the contrary, a toner is too charged when not fed enough and the resultant image does not have enough density. Therefore, a conventional image forming apparatus includes a toner concentration sensor detecting a toner concentration in a two-component developer to control an amount of the toner fed thereto so as to have a target concentration of the two-component developer in an image developer.
It is known that a toner concentration required to produce images having a desired image density depends on an environmental conditions such as temperature and humidity of the developer and usage conditions thereof. Therefore, image density control to determine a target toner concentration suitable to the present conditions is made at a predetermined timing in many cases. In this image control, a toner pattern having a predetermined image density is formed on an image bearer, and the target toner concentration is amended, based on the result of the toner adherence detected by the image density sensor. Thus, the target toner concentration suitable to the present conditions is set and images having desired image density are stably produced.
Most of conventional toners include a binder resin which is a mother toner and a release agent such as a wax. However, when such a two-component developer including a toner including a release agent receives a mechanical stress such as stir and development process pressure, the toner agglutinates. Particularly, in a high-speed machine having a system speed, i.e., a surface traveling speed (process linear speed) of a latent image bearer of from 400 to 1,700 mm/sec, a stirrer stirs the two-component developer at high speed as well in an image developer, a toner receives a large mechanical stress and the toner more agglutinates.
When a toner more agglutinates in a two-component developer in an image developer, a difference between a toner concentration detected by the toner concentration and an actual toner concentration in the two-component developer becomes large and detection preciseness deteriorates. Therefore, the toner concentration is controlled based in an erroneous detection result, the actual toner concentration is excessively high or low, resulting in production of abnormal images. This problem is required to solve in such high-speed machines in which a toner tends to agglutinate.
Most conventional MFPs have a system linear speed not faster than 400 mm/sec, which is slow and a developer including a toner receives less mechanical stress in a developing unit. Therefore, a wax locally present on the surface of a toner does not noticeably agglutinate the toner. However, a digital printer used in on-demand printing in compliance with printing needs such as printmaking less, and volume less and varying documents is required to have a system linear speed not less than 400 mm/sec, and therefore a developer including a toner receives a large mechanical stress in a developing unit. A wax locally present on the surface of a toner agglutinates the toner and the developer deteriorates in fluidity, and a difference between a toner concentration detected by the toner concentration and an actual toner concentration in the two-component developer becomes larger, resulting in toner scattering.
Because of these reasons, a need exits for a high-speed image forming apparatus preventing production of abnormal images due to deterioration of toner concentration detection preciseness caused by toner agglutination.